Karol J. Mysels

The surface tension of sodium dodecylsulfate solutions and the phase separation model of micelle formation

Abstract A method for obtaining accurate and precise surface tension measurements of surfactant solutions above the c.m.c. is described. It depends on purification by foaming below the c.m.c., followed by fractionation into a more concentrated and more dilute solution. Data are obtained as the concentration is varied by recombining the two solutions until a return to the initial value shows the absence of contamination during the whole operation, which is performed in a compact apparatus. The effect of typical contaminants was also studied and their complete removal ascertained. A modified equilibrium Wilhelmy method was used and its accuracy is discussed. The results show a significant decrease of the surface tension of solutions of sodium dodecyl sulfate above the c.m.c. proving that its activity increases in this range. This finding places a definite limit upon the validity of the approximation represented by the phase separation theory of micelle formation. On the other hand, another approximation, a simple mass action calculation, accounts remarkably well for the data. Measurements below the c.m.c. obtained by essentially the same method indicate a significant increase of adsorption throughout this region.

Direct measurement of the variation of double-layer repulsion with distance

Soap films can be subjected to compressive stresses exceeding 1 atm while their thickness is measured optically in an apparatus which is described in detail. It permits forming the film within a ring of porous porcelain whose pores communicate to the outside, whereas the film is in an enclosure in which the air pressure can be varied. The applied pressure is balanced primarily by the double-layer repulsion between the monolayers of the film. Hence as the pressure is increased, the film thickness decreases showing how double-layer repulsion varies with the distance between the monolayers. The agreement with theory is satisfactory as far as it pertains to the region of low potentials which determines the slopes of the distance dependence. The absolute values involve the less certain high potential region of the theory as well as assumptions about the structure of the films but can be brought into reasonable agreement. The effect of van der Waals forces can also be seen at higher ionic strengths.

Conductivity of mixed sodium decyl and dodecyl sulfates—the composition of mixed micelles☆

Abstract Precise measurements of conductivity of solutions of pure sodium decyl and dodecyl sulfates and their mixtures in the region of the critical micelle concentration (c.m.c.) are reported and the c.m.c. values of the mixed systems are evaluated. An empirical interpretation of these results is proposed which leads to a self-consistent assignment of composition to the mixed micelles. This interpretation is based on an extrapolation—on the conductivity vs. concentration plot—to obtain the concentration of monomers in a mixed solution in the same way in which the c.m.c. is obtained in a pure system. This extrapolated concentration is assumed to be the c.m.c. of the mixed monomers, again in analogy with the pure system. This concentration therefore determines the composition of the equilibrium monomers. The composition of the mixed micelles is then calculated by difference.

Charge effects in light scattering by association colloidal electrolytes

Abstract A simplified treatment of the effect of charge on the light scattering by colloidal solutions is extended to take into account the refractive index contribution of simple electrolyte. It is shown that this effect leads to a reduction of optical efficiencies of fluctuations and to an apparently smaller molecular weight. In case of association colloidal electrolytes the effect of monomeric ions as well as that of added electrolytes is taken into account. The effect of equilibrium between micelles and monomeric ions on the concentration of monomer is examined under idealized conditions and leads to a minor correction. Literature data on micellar weights are recalculated in the light of these ideas.

Thermodynamic aspects of mixed micelles—application to an empirically established equilibrium

Abstract Relationships between the overall composition—in terms of mole fractions of two micelle-forming components—in a solution of these two components and the critical micelle concentration on the one band and the composition of the mixed micelles on the other, are reviewed and activity coefficients introduced to account for the behavior of real systems. It is shown that the behavior of the critical micelle concentration is not a sufficient criterion for the ideality of the system, that the contribution to nonideality of the two components can be resolved, and that the ideality of the monomeric solution may be tested if the composition of mixed micelles is known. These considerations are applied to the results obtained in the preceding paper for the sodium decyl and dodecyl sulfates and indicate definite deviations from ideality for the whole system as well as for the monomeric solution. These deviations are, however, not very large and affect the mixed critical micelle concentration insignificantly.

Electrodiffusion : A Fluctuation Method for Measuring Reaction Rates

A new approach to the problem of measuring rapid ionic reactions is proposed. It is based on the fact that if a dynamic equilibrium system involves a species changing between two ionic forms at a rapid rate, the times spent by the individual particles in the two forms fluctuate randomly. In an electric field this produces a random walk corresponding to an electrically induced diffusion coefficient. The extent of this electrodiffusion is correlated with the rate of the reaction. It is shown that in principle it is feasible to measure reaction rates down to half‐lives of the order of ten milliseconds, and that the use of alternating current, and particularly of reversing direct current, offers distinct advantages.

An experimental test of frankel's law of film thickness☆

Abstract Experimental methods for determining the relation between thickness and velocity of pull-out for surfactant films are presented. When applied to a variety of surfactant systems over a wide range of bulk viscosities, purity, and surface viscosity, they yield data in excellent agreement with Frankels law, which relates film thickness and velocity of pull-out to bulk viscosity, density, surface tension, and the force of gravity on the basis of simple hydrodynamics assuming negligible surface extensibility in the region of interest.

Conductimetric determination of the critical micelle concentration of surfactants in salt solutions

Abstract The conductimetric method of determining the critical micelle concentration of ionic surfactants has been used at very high ionic strengths. Data on sodium decyl and dodecyl sulfates in the presence of up to 1.0 and 0.3 N NaCl, respectively, were obtained. The problems involved in these determinations are discussed. The principal one is the unexpected equality of the equivalent conductances of micelles and monomers in the presence of about 0.35 N NaCl.

Tracer electrophoresis III. The mobility of the Na+ constituent in sodium lauryl sulfate solutions

Abstract The concentration of free LS− ions appears to be approximately constant and the degree of ionization of micelles seems to increase slowly with concentration. These results of mobility measurements using radio Na+ tracer and a model of a single type of micelle are in agreement with much previous work and thus provide a confirmation of theories of relaxation effects in electrophoresis of large particles. On the other hand, the retrograde ionization of micelles remains very puzzling.

Electric conductivity of sodium decyl sulfate solutions

Abstract A modified method of preparing the sodium decyl sulfate based on the insolubility of its alcoholate is described, and conductivity measurements in the highly dilute region and in the c.m.c. region in water and in the presence of salt are reported. The behavior of this compound seems quite normal throughout and it shows no indication of marked dimerization.